This invention relates to marine seismic exploration, and more particularly, relates to determining the positions of towed marine seismic sources, towed marine seismic detectors, and other towed marine apparatus.
In marine seismic exploration, various impulsive sources, which are typically air guns, may be suspended at some preselected depth beneath a float, or some other type of support apparatus. The float or apparatus is in turn towed by an exploration vessel; there may be a plurality of such floats or apparatus towed behind the exploration vessel. The exploration vessel may also tow a streamer cable, or another exploration vessel may tow the streamer cable. The streamer cable contains detectors to detect energy propagating upwardly from subsurface strata lying beneath the body of water in which the vessel operates.
The exploration vessel may determine its precise location in the body of water through the use of conventional navigation systems. Although such systems may be employed to determine the vessel's location, they generally do not determine the location of any source floats or apparatus, or streamer cable. Other onboard equipment must be employed to determine the location of the floats or apparatus, or streamer cable that may be in many different positions and at various distances depending upon the source and/or detector arrays employed.
Recently developed seismic prospecting techniques for so-called 3-D shooting, or for improving signal-to-noise ratios from subsurface layers, employ a plurality of seismic sources spaced from each other. These plurality of seismic sources may be deployed in either wide or long arrays. These wide or long source arrays require a plurality of floats or apparatus appropriately disposed in the water and towed by the exploration vessel.
The exploration vessel must determine the location of these plurality of sources and the streamer cable, as well as any other types of towed detectors, in order to accurately process any resulting detected signals. Further, it is important to know the location of such towed seismic apparatus during vessel turns to avoid damage to the apparatus. Thus, there is currently an unfulfilled need for simple onboard apparatus to locate towed seismic apparatus.
These and other limitations and disadvantages are overcome by the present invention, however, and methods and apparatus are provided for determining the positions of the towed seismic apparatus relative to the towing vessel.